Heterogeneous delivery of oxygen to tissues plays a crucial role in many biological processes and the treatment of many diseases. This proposal describes the use of a novel oxygen microelectrode device to measure the effect of localized oxygen delivery on tumor necrosis. The specific goals are to: 1) develop an oxygen microgradient chip that allows measurement and control of the oxygen environment in a tissue with microscale spatio-temporal resolution; 2) precisely deliver oxygen to tumor tissue, measure individual cell death, and determine the extent that poor oxygenation causes tumor necrosis; and 3) demonstrate that a HIF1-alpha response is critical for cell survival in hypoxic tumor tissue. Tumor hypoxia and necrosis are both caused by poor oxygen supply and are both correlated with poor patient prognosis. Hypoxia inducible factor1-alpha (HIF1-alpha) is a promising therapeutic target because it promotes cell survival in hypoxic conditions, which may lead to tumor growth. No technology currently exists that allows control of the oxygen microenvironment of tissues with cellular resolution. The proposed device will generate stable microgradient gas profiles that are responsive to transient experimental conditions. This device will be useful with a wide range of tissues and applications including tissue engineering and stem cell differentiation. In this proposal, oxygen will be delivered to HIF1-a-containing and null in vitro tumors while preserving other concentration gradients (glucose, acidic waste products, etc.). By measuring the extent of cell death, the effect of oxygen gradients on cell survival will be independently discerned. Many promising therapeutics are being designed to target the unique microenvironments of tumors. Here, precisely controlled experiments will evaluate the therapeutic potential of targeting both hypoxia and HIF1-alpha. In summary, microelectrodes will be used to deliver oxygen to three-dimensional tumor tissue to measure the effect of oxygen on cell survival and HIF1-alpha expression.